66 valiant with newer slant six

You will need to check the end of the crank to see if it's been machined for a stock pilot bushing. If not, use the later magnum pilot BEARING that will fit in the converter register. The difference on the flywheel will be the center hole diameter. Just get the '68-up flywheel for the newer engine.
Thanks for the info man. Trying to keep notes for when I get ready to get a motor to rebuild.